ISLAMABAD, Pakistan (CNN) -- Pakistan intensified its military operation to regain control of the country's northwest from Taliban militants, with fighting focused on the largest city in the Swat Valley on Sunday.

The campaign in Pakistan's northwest has created a growing humanitarian crisis.

Troops were engaged in street battles with militants in Mingora, the gateway into the valley.

About 15,000 to 20,000 civilians remain in the city, but the fighting has not led to civilian casualties, said military spokesman Maj. General Athar Abbas. Tens of thousands of others have fled.

Security forces cleared and secured two important areas in Mingora, he said. One of them is the power station and authorities hope to restore power to the town soon, he added.

The ground offensive in Mingora is part of the military's vow to flush out the Taliban from areas in the North West Frontier Province. The militants have long controlled vast swaths of the province, imposing their own fundamentalist interpretation of Islamic law.

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Swat Valley, which is close to the border with Afghanistan, was once one of Pakistan's biggest tourist destinations. However the central government has long exerted little control in the area.

As retaliation for the government's military presence, the Taliban carried out a series of deadly attacks, beheadings and destruction of girls' schools.

Earlier this year, Islamabad struck a peace deal with the Taliban. As part of the pact the Taliban was allowed to impose sharia, or Islamic law, in the valley, which among other restrictions, prevents women from being seen in public without their husbands or fathers.

But the deal soon fell apart after the militants took control of the neighboring Buner district.

Control of Buner brought the Taliban closer to the capital, Islamabad, than it had been since it mounted its insurgency. The capital of the nuclear-armed nation is just 96 km (60 miles) from Buner.

The militants' move into Buner prompted the military's latest offensive.

On Friday, the army said it has broken the backs of the Taliban in the valley and was clearing them out city by city.